Polishing composition

ABSTRACT

A polishing composition of abrasive grains, a phosphorus-containing inorganic acid or salt thereof and another inorganic acid or salt thereof contained in an aqueous medium is used for the polishing of magnetic disk substrates or the like.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is an application filed under 35 U.S.C. § 111(a)claiming the benefit pursuant to 35 U.S.C. § 119 (e) (1) of the filingdate of Provisional Application Ser. No. 60/317,962 filed on Sep. 10,2001 pursuant to 35 U.S.C. § 111(b).

TECHNICAL FILED

The present invention relates to a polishing composition suited to thepolishing of a magnetic disk substrate used in a storage device for acomputer or the like, and more particularly to a composition forpolishing a magnetic disk substrate that can provide a magnetic disksurface polished with high precision suitable for when a magnetic headflies over the disk surface while floating at a low level.

BACKGROUND ART

Among the external storage devices for a computer or word processor, themagnetic disk (memory hard disk) is widely used as a means of performinghigh-speed access. One typical example of this magnetic disk is a diskobtained by subjecting the surface of an Al-alloy substrate toelectroless plating with NiP to form a substrate, polishing the surfaceof the substrate, and successively sputtering a Cr-alloy undercoatlayer, a Co-alloy magnetic layer and a carbon protective layer on thesubstrate in that order.

If a protrusion of such height so as to exceed the floating level withthe magnetic head remains on the magnetic disk surface, the magnetichead which flies at high speed while floating at a stipulated heightabove the disk surface may collide with the protrusion, thereby causingdamage. In addition, when a protrusion or a scratch caused by polishingis present on the magnetic disk substrate, the protrusion also appearson the Cr-alloy undercoat layer and the Co-alloy magnetic layer whensuch layers are overlaid, and a flaw caused by the scratch is produced,whereby the magnetic disk surface does not have a high-precision smoothsurface. Accordingly, it is necessary to polish the substrate preciselyto enhance the precision of the disk surface.

For the polishing of the magnetic disk substrate, many polishingcompositions are proposed that can remove the protrusions completely orbuff the protrusion down to a height as low as possible, without easilyproducing any scratches.

The JP-A-HEI 9-204657 discloses the use of a composition prepared byadding aluminum nitrate and an anti-gelling agent to colloidal silica.JP-A-HEI 9-204658 discloses the use of a composition prepared by addingaluminum nitrate to fumed silica. Each of the compositions disclosed inthese publications comprises finely divided particles of silicon oxidewith low hardness serving as the abrasive grains, so that good surfaceprecision can be easily obtained although it is difficult to achieve apolishing speed suitable for actual production.

Furthermore, in JP-A-HEI 10-204416, the use of many kinds of oxidizingagents and the use of Fe salts are independently proposed to acceleratethe polishing speed. However, the polishing speed thus obtained is stillinsufficient in light of actual production in practice.

A composition for polishing an aluminum magnetic disk substrate thatpermits high-density magnetic storage is required to provide a disksurface with high precision that enables a head to fly at a low level.

Accordingly, an object of the present invention is to provide acomposition for polishing a magnetic disk substrate capable of realizinghigh-density magnetic storage, where such composition can give themagnetic disk substrate a low surface roughness with no protrusions orscratches caused by polishing and can polish the magnetic disk substrateat a cost-effective speed.

DISCLOSURE OF THE INVENTION

The polishing composition according to the present invention comprises:

abrasive grains, a phosphorus-containing inorganic acid or salt thereofand another inorganic acid or salt thereof contained in an aqueousmedium.

Furthermore, the abrasive grains are at least one selected from thegroup consisting of alumina, titania, silica and zirconia.

Moreover, the mean particle size of the abrasive grains is from 0.001 to0.5 μm and the abrasive grains are colloidal particles.

In addition, the phosphorus-containing inorganic acid is phosphoric acidor phosphonic acid, the other inorganic acid is at least one acidselected from the group consisting of nitric acid, sulfuric acid,amidosulfuric acid and boric acid.

Furthermore, the oxidizing agent is at least one compound selected fromthe group consisting of the peroxides, perborates, persulfates ornitrates, the peroxide is hydrogen peroxide and the perborate is sodiumperborate.

In addition, the pH of the polishing composition is 1 to 5.

Moreover, the abrasive grain content is in the range from 3 to 30% bymass, the content of the inorganic acid or salt thereof is in the rangefrom 0.1 to 8% by mass and the oxidizing agent content is in the rangefrom 0.2 to 5% by mass.

In addition, the polishing composition according to the presentinvention is a composition for polishing magnetic disk substrates.

The present invention also comprises a magnetic disk substrate polishedby the aforementioned composition for polishing magnetic disksubstrates.

As described above, the polishing composition according to the presentinvention comprises two or more specific organic acids or salts thereof,and thus the state of dispersion becomes good, and by taking aphosphorus-containing organic acid or salt thereof as a mandatoryconstituent, the effects of increasing the polishing rate andsuppressing the occurrence of micro-scratches are boosted, so a magneticdisk substrate that has a high-precision disk surface is obtained.

BEST MODE FOR CARRYING OUT THE INVENTION

The inventors of the present invention have diligently investigated anabrasive that can provide a polished surface with high precisionrequired for an aluminum magnetic disk for use with a low-flyingmagnetic head. As a resulti the inventors have found a polishingcomposition that exhibits excellent properties in polishing an aluminummagnetic disk, leading to the present invention.

The polishing composition of the present invention is characterized inthat abrasive grains, a phosphorus-containing inorganic acid or saltthereof and another inorganic acid or salt thereof are contained in anaqueous medium.

The abrasive grains contained as an abrasive for use in the polishingcomposition of the present invention are not particularly limited. Forexample, alumina, titania, silica, zirconia and the like can beemployed, and the crystalline form thereof is not limited. For instance,alumina (i.e., aluminum oxide) has the α, γ, δ, η, θ, κ, χ and othercrystalline forms; titania (i.e., titanium oxide) has the rutile,anatase, brookite and other crystalline forms; silica (i.e., siliconoxide) includes the colloidal silica, fumed silica, white carbon andother forms; and zirconia (i.e., zirconium oxide) has the monoclinic,tetragonal and amorphous forms. Any of these can preferably be used. Theabrasive grains in the form of colloidal particles preferably boostsuppressing the occurrence of micro-scratches.

The aforementioned abrasive grains have a mean particle size normally inthe range of 0.001 to 0.5 μm, preferably in the range of 0.001 to 0.2μm, more preferably in the range of 0.02 to 0.2 μm, and most preferablyin the range of 0.03 to 0.2 μm. In addition, abrasive grains in the formof colloidal particles are even more preferable. Here, the mean particlesize is used is a value measured by a Microtrac UPA150 (made byHoneywell, Inc.) laser Doppler frequency analysis-type particle sizedistribution analyzer.

As the particle size of the abrasive grains increases, gelling andaggregation of fine particles are more easily suppressed, but theprobability of the presence of course grains increases, thereby causingscratches to occur in the course of polishing. Conversely, as theparticle size of the abrasive grains decreases, the aforementionedgelling and aggregation occur more easily and this also causes scratchesto occur in the course of polishing.

When the abrasive grain concentration within the polishing compositionof the present invention is less than 3% by mass (hereinafter indicatedby simply “%” unless otherwise noted), the polishing speed issignificantly decreased. As the abrasive grain concentration increases,the polishing speed increases, but when the abrasive grain concentrationexceeds 30%, not only is no increase in polishing speed seen, butgelling takes place readily, particularly when the abrasive grains arein the form of colloidal particles. In view of cost efficiency, an upperlimit for the concentration is 30% for practical use. Accordingly, theconcentration of the abrasive grains in the polishing composition ispreferably in the range of 3 to 30% and more preferably 5 to 15%.

In the polishing composition of the present invention, both aphosphorus-containing inorganic acid or salt thereof and anotherinorganic acid or salt thereof are used. The phosphorus-containinginorganic acid must contain phosphorus as a constituent element of thecompound, and is preferably phosphoric acid or phosphonic acid. Thephosphorus-containing inorganic acid also comprises derivatives thereof.Two or more phosphorus-containing inorganic acids may also be usedtogether.

The other inorganic acid used mixed with the phosphorus-containinginorganic acid may be hydrochloric acid, sulfuric acid, chromic acid,carbonic acid, amidosulfuric acid, boric acid or other acid, but nitricacid, sulfuric acid, amidosulfuric acid or boric acid is preferable.These acids also comprise derivatives thereof The salts of thephosphorus-containing inorganic acid or salt thereof and the anotherinorganic acid or salt thereof used may be salts of Li, Be, Na, Mg, K,Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Al, Zr, Nb, Mo, Pd, Ag, Hf, Ta, Wor other metals. These salts may be obtained by dissociating an oxide orcarbonate of the aforementioned metals, for example, in aphosphorus-containing inorganic acid or another inorganic acid.

In the polishing composition of the present invention, the total contentof the phosphorus-containing inorganic acid or salt thereof and anotherinorganic acid or salt thereof may be in the range from 0.1 to 8%,preferably 0.2 to 6% and more preferably 0.4 to 4%. When content ofinorganic acid or salt thereof is less than 0.1%, no effect ofsuppressing micro-scratches or increasing the polishing speed is seen.In excess of 8%, the drop in pH is significant, thus leading to greatdamage to the polishing material so this causes problems in handling.The proportion of mixing the phosphorus-containing inorganic acid orsalt thereof and another inorganic acid or salt thereof is preferably:to 1 mole of the former is added a range of 0.1 to 5 moles of thelatter.

If the latter is less than 0.1 moles, then the dispersibility in slurrydeteriorates and the micro-scratches increase. If 5 moles is exceeded,the pH drops and the damage to the polishing material increases.

The polishing composition according to the present invention comprisestwo or more specific organic acids or salts thereof, but by taking aphosphorus-containing organic acid or salt thereof as a mandatoryconstituent, the effects of increasing the polishing rate andsuppressing the occurrence of micro-scratches are boosted.

In the present invention, the mechanism behind the effect of using twoor more specific organic acids or salts thereof is not certain, but oneis presumed to be because the state of dispersion as a polishingcomposition becomes better.

The oxidizing agent contained in the polishing composition of thepresent invention is preferably at least one compound selected from thegroup consisting of the peroxides, perborates, persulfates or nitrates,where representative examples include: hydrogen peroxide as a peroxide,sodium perborate as a perborate, ammonium persulfate as a persulfate andammonium nitrate as a nitrate. Note that if the aforementioned otherinorganic acid or salt thereof has an oxidizing action, then it can alsoserve as the oxidizing agent, so it is possible to use that inorganicacid or salt thereof alone. For example, a nitrate may be used both asthe salt of the other acid and as the oxidizing agent. Note that even inthe case that the other inorganic acid or salt thereof has an oxidizingaction, a different oxidizing agent may also be added.

Examples of the effects of the oxidizing agent include increasing thepolishing rate and reducing the surface roughness. While the mechanismsof these effects are not clear, they are thought to be the effect as anetching agent on the NiP surface.

The content of the oxidizing agent (i.e., hydrogen peroxide) should be0.2 to 5% or preferably 0.5 to 2%. If the oxidizing agent content isless than 0.2% then the effect of increasing the polishing rate andreducing the surface roughness is minuscule, but if 5% is exceeded thenits effect reaches saturation.

The content of the other inorganic acid or salt thereof in the eventthat it is also used as the oxidizing agent becomes the content of both,namely the content as the other inorganic acid or salt thereof and thecontent as the oxidizing agent. Accordingly, the sum of the content ofthe phosphorus-containing inorganic acid or salt thereof and the otherinorganic acid or salt thereof is preferably 0.3 to 13% or morepreferably 0.7 to 8%.

Moreover, in the case that the aforementioned composition containsinorganic acids or salts thereof not also used as oxidizing agents, itis possible to include one or the other of the also used inorganic acidor salts thereof to the maximum limit, and then determine the upperlimit for the content of the remaining oxidizing agent not also used.For example, in the case of phosphoric acid, a nitrate (a salt ofanother inorganic acid and also an oxidizing agent) and hydrogenperoxide, the content of the nitrate may be determined as all being theother inorganic acid, while the hydrogen peroxide may be taken as theoxidizing agent and its upper limit can be set to 5%. In addition, inthe case that an inorganic acid or salt thereof that is not an oxidizingagent such as a nitrate for example is included, the nitrate is includedas an oxidizing agent with 5% as the limit, and the amount of theremaining non-oxidizing inorganic acid or salt thereof can bedetermined. In either case, the sum of the content of thephosphorus-containing inorganic acid or salt thereof, the otherinorganic acid or salt thereof and the oxidizing agent is preferably 0.3to 13% and more preferably 0.7 to 8%.

In the polishing composition of the present invention, the pH range ispreferably 1 to 5, more preferably 2 to 4 and even more preferably 2 to3. Making the liquid acidic can accelerate the oxidation of Ni andincrease the polishing rate, but if the pH is too low, corrosion ofequipment and other problems occur, so the pH is most preferably 2 to 3.Adjustment of the pH can be performed using sodium perborate.

Note that the concentrations of the various aforementioned componentsare the concentrations at the time of polishing the magnetic disksubstrate. In the case that a polishing composition is transported afterbeing produced, it is advantageous to prepare a composition with aconcentration higher than that described above and dilute it to theaforementioned concentrations before use.

The magnetic disk substrate polishing composition according to thepresent invention may further comprise surfactants and preservatives inaddition to the aforementioned components. However, close attention mustbe paid with respect to their type and content so that gelling does notoccur.

An anti-gelling agent may be added to the polishing composition in orderto suppress gelling. As the anti-gelling agent that may be used, atleast one compound selected from the group consisting of a phosphonicacid compound, phenanthroline and aluminum acetylacetonate is preferablyemployed. Specific examples of the phosphonic acid compound includephosphoric acid, 1-hydroxyethane-1,1-diphosphonic acid (C₂H₆O₇P₂) andaminotrimethylenephosphonic acid (C₂H₁₂O₉P₃N). 1,10-phenanthrolinemonohydrate (C₁₂H₈N₂.H₂O) and an aluminum complex salt of acetylacetone(Al₂[CH(COCH₃)₃]) are respectively given as examples of thephenanthroline compound and the aluminum acetylacetonate. It ispreferable that the anti-gelling agent be added to the polishingcomposition in an amount of 2% or less.

The polishing composition of the present invention can be prepared bysuspending the abrasive grains in water and adding thephosphorus-containing inorganic acid or salt thereof, nitrate-containinginorganic acid or salt thereof and hydrogen peroxide to the suspensionin a manner similar to that used in the preparation of conventionalpolishing compositions. All the components may be mixed and diluted foruse. Alternatively, there is a method of preliminarily dividing thecomponents into two groups, for example, one including water, abrasivegrains and nitric acid, and the other including water, phosphoric acidand hydrogen peroxide, and thereafter mixing the two groups together.

The polishing composition of the present invention is advantageouslyapplicable to a substrate of a magnetic disk with a high storage density(generally, with a storage density of 3 Gbits/inch² or more), asrepresented by a magnetic disk for a magnetic head utilizing amagnetoresistance (MR) effect. Application to a magnetic disk with alower storage density is also effective from the standpoint of improvingreliability.

The magnetic hard disk substrates to which the polishing composition ofthe present invention is applicable are not particularly limited, butwhen the polishing composition of the present invention is applied to analuminum substrate (including an aluminum alloy substrate), andparticularly an aluminum substrate that is plated with NiP for exampleby electroless plating, a high-quality polished surface can beadvantageously obtained from an industrial standpoint.

The polishing method typically comprises the steps of causing apolishing pad commonly used for a slurry-like abrasive to press againstthe magnetic disk substrate, and rotating the pad or the substrate whilesupplying the slurry to a gap between the pad and the substrate.

Magnetic disks made from substrates polished using the polishingcomposition of the present invention exhibit an extremely low frequencyof occurrence of fine defects such as micro-pits and micro-scratches,and the surface of the magnetic disk has excellent surface smoothness,indicated by a surface roughness (Ra) of roughly 2 to 3 μm.

Here follows a detailed description of the present invention made withreference to examples, but the present invention is in no way limited tothese examples.

WORKING EXAMPLES 1 TO 15

Table 1 below shows the kinds of abrasives and the properties thereofused in the respective Working Examples and Comparative Examples. TABLE1 Abrasive grain (Name Mean particle used in Table 2) Trade name size(μm) Silicon oxide [1] (Silica [1]) Syton HT-50 0.05 Silicon oxide [2](Silica [2]) E-150J 0.1 Silicon oxide [3] (Silica [3]) Aerosil 50 0.1Silicon oxide [4] (Silica [4]) Snowtex 30 0.01 Titanium oxide [1](Titania [1]) F-2 0.3 Titanium oxide [2] (Titania [2]) F-4 0.2 Alumina0.2 Zirconia 0.2

Water, inorganic acid 1, inorganic acid 2 and an oxidizing agent wereadded to colloidal silica (Syton HT-50F) made by DuPont in theproportions indicated in Table 2 to prepare a variety of aqueouspolishing compositions. Polishing was performed with a polishing machineunder the polishing conditions given below. The results are shown inTable 2.

The mean particle size was measured with a Microtrac UPA150 (made byHoneywell, Inc.) laser Doppler frequency analysis-type particle sizedistribution analyzer. The measured values of the grain size are shownin Table 1. The pH of the composition was measured using a Horiba, Ltd.D-13 hydrogen ion concentration meter with glass electrodes.

WORKING EXAMPLES 16 AND 17

A mixture of white carbon (E-150J) made by Nippon Silica Industrial Co.,Ltd. and fumed silica (Aerosil 50) made by Nippon Aerosil Co., Ltd. wasground in a stirred mill and course particles were removed byclassification, thereby obtaining silicon oxide with a mean particlesize of 0.1 μm. Next, water, inorganic acid 1, inorganic acid 2 and anoxidizing agent were added in the proportions indicated in Table 2 toprepare a variety of aqueous polishing compositions. Polishing wasperformed with a polishing machine under the polishing conditions givenbelow. The results are shown in Table 2.

WORKING EXAMPLES 18 TO 20

Titanium oxide (Supertitania F-4) made by SHOWA TITANIUM CO., LTD.,alumina and zirconia were ground in a stirred mill and course particleswere removed by classification, thereby obtaining titanium oxide,alumina and zirconia with a mean particle size of 0.2 μm. Next, water,inorganic acid 1, inorganic acid 2 and an oxidizing agent were added inthe proportions indicated in Table 2 to prepare a variety of aqueouspolishing compositions. Polishing was performed with a polishing machineunder the polishing conditions given below. The results are shown inTable 2.

WORKING EXAMPLE 21

To colloidal silica (Snowtex 30) made by Nissan Chemical Industries,Ltd., water, inorganic acid 1, inorganic acid 2 and an oxidizing agentwere added in the proportions indicated in Table 2 to prepare a varietyof aqueous polishing compositions. Polishing was performed with apolishing machine under the polishing conditions given below. Theresults are shown in Table 2.

(Polishing conditions)

As a substrate, a 3.5-inch aluminum disk plated with NiP by electrolessplating was employed.

(Polishing machine and polishing conditions)

-   -   Polishing machine . . . 4-way double-sided polishing machine    -   Polishing pad . . . Suede type (Polytex DG, made by Rodel, Inc.)    -   Lower surface plate speed 60 rpm    -   Slurry feed rate . . . 50 ml/min    -   Polishing time . . . 5 min    -   Working pressure . . . 50 g/cm²        (Evaluation of polishing properties)    -   Polishing rate . . . Calculated from the decrease in weight of        the aluminum disk after polishing    -   Surface roughness Measured using Talystep and Talydata 2000        (made by Rank Taylor Hobson Co.)

The depth of polishing scratches was found using morphology analysiswith a Tencor P-12 stylus-type surface analyzer in 3D mode.

Table 2 shows the results of evaluation of the polishing properties. Thesymbol A in the “Scratches” column in Table 2 indicates a polishingscratch depth of 2 nm or less. The symbol B in the “Scratches” columnindicates a polishing scratch depth of 2-10 nm. No polishing scratchdepths greater than 10 nm were found in either the Working Examples orComparative Examples.

COMPARITIVE EXAMPLES 1 TO 7

Water, aluminum nitrate, etc., and hydrogen peroxide were added tocolloidal silica (Syton HT-50F) made by DuPont in the proportionsindicated in Table 3 to prepare aqueous polishing compositions.Polishing was performed in the same manner as in the Working Examples.The results are shown in Table 3.

COMPARITIVE EXAMPLE 8

Titanium oxide (Supertitania F-2) made by SHOWA TITANIUM CO., LTD. wasground in a stirred mill and course particles were removed byclassification, thereby obtaining titanium oxide with a mean particlesize of 0.3 μm. Next, water and aluminum nitrate were added in theproportions indicated in Table 3 to prepare an aqueous polishingcomposition. Polishing was performed in the same manner as in theWorking Examples. The results are shown in Table 3.

COMPARATIVE EXAMPLE 9

Titanium oxide (Supertitania F-4) made by SHOWA TITANIUM CO., LTD. wasground in a stirred mill and course particles were removed byclassification, thereby obtaining titanium oxide with a mean particlesize of 0.2 μm. Next, water and aluminum nitrate were added in theproportions indicated in Table 3 to prepare an aqueous polishingcomposition. Polishing was performed in the same manner as in theWorking Examples. The results are shown in Table 3. TABLE 2 Inorganicacid Polish- Surface Work- Abrasive grains 1 2 Oxidizing agent ingrough- ing Content content content Content rate ness Exam- (% by (% by(% by (% by (μm/ Ra ple Type mass) Type mass Type mass Type mass) pHmin) (nm) Scratches Pits 1 Silica 5 Phosphoric acid 1.5 Nitric acid 0.4H₂O₂ 0.5 1.3 0.18 0.2 A A [1] 2 Silica 10 ″ 1.5 ″ 0.4 ″ 0.5 1.4 0.21 0.2A A [1 ] 3 Silica 15 ″ 1.5 ″ 0.4 ″ 0.5 1.6 0.22 0.2 A A [1 ] 4 Silica 10″ 0.5 ″ 0.4 ″ 0.5 1.6 0.19 0.2 A A [1 ] 5 Silica 10 ″ 3.0 ″ 0.4 ″ 0.51.2 0.22 0.2 A A [1 ] 6 Silica 10 ″ 1.5 ″ 0.2 ″ 0.5 1.6 0.19 0.2 A A [1] 7 Silica 10 ″ 1.5 ″ 0.6 ″ 0.5 1.2 0.22 0.2 A A [1 ] 8 Silica 10 ″ 3.0″ 0.4 ″ 0.3 1.4 0.19 0.2 A A [1 ] 9 Silica 10 ″ 3.0 ″ 0.4 ″ 2.0 1.4 0.220.2 A A [1 ] 10 Silica 10 ″ 1.5 Amidosulfuric 0.4 ″ 0.5 1.6 0.20 0.2 A A[1] acid 11 Silica 10 ″ 1.5 Sulfuric acid 0.4 ″ 0.5 1.4 0.20 0.2 A A [1] 12 Silica 10 ″ 1.5 Boric acid 0.4 ″ 0.5 1.5 0.20 0.2 A A [1 ] 13Silica 10 ″ 1.5 Nitric acid 0.4 NaBO₃ 1.5 1.7 0.21 0.2 A A [1 ] 14Silica 10 ″ 1.5 ″ 0.4 Ammonium 2.0 1.2 0.20 0.2 A A [1] persulfate 15Silica 10 Phosphonic 1.5 ″ 0.4 H₂O₂ 0.5 1.4 0.21 0.2 A A [1] acid 16Silica 10 Phosphoric acid 1.5 ″ 0.4 ″ 0.5 1.4 0.20 0.2 A A [2] 17 Silica10 ″ 1.5 ″ 0.4 ″ 0.5 1.4 0.20 0.2 A A [3] 18 Titania 6 ″ 1.5 ″ 0.4 ″ 0.51.4 0.21 0.3 A A [2] 19 Alu- 15 ″ 1.5 ″ 0.4 ″ 0.5 1.4 0.21 0.3 A A mina20 Zirconia 15 ″ 1.5 ″ 0.4 ″ 0.5 1.4 0.21 0.3 A A 21 Silica 10 ″ 1.5 ″0.4 ″ 0.5 1.4 0.14 0.1 A A [4]

TABLE 3 Inorganic acid Polish- Surface Compara- Abrasive grains 1 2Oxidizing agent ing rough- tive Content Content Content Content rateness Exam- (% by (% by (% by (% by (μm/ Ra ple Type mass) Type mass Typemass Type mass) pH min) (nm) Scratches Pits 1 Silica [1] 10 Aluminumnitrate 5.0 — H₂O₂ 1.0 2.0 0.08 0.4 B A 2 ″ 10 — — ″ 1.0 2.0 0.09 0.2 AA 3 ″ 10 Phosphoric 3.0 — ″ 1.0 1.5 0.11 0.2 A A acid 4 ″ 10 Phosphonic3.0 — ″ 1.0 1.5 0.11 0.2 A A acid 5 ″ 10 Nitric acid 0.4 — ″ 1.0 1.50.11 0.2 A A 6 ″ 10 Amidosulfuric 1.0 — ″ 1.0 2.0 0.11 0.2 A A acid 7 ″10 Sulfuric acid 0.4 Amidosulfuric 0.4 ″ 1.0 1.3 0.11 0.2 A A acid 8Titania [1]  6 Aluminum nitrate 5.0 — — 3.0 0.21 0.4 B B 9 Titania [2] 6 Aluminum nitrate 5.0 — — 3.0 0.21 0.3 B B

Industrial Applicability

When a disk is polished using the polishing composition according to thepresent invention, the disk can be quickly polished so that the surfaceroughness becomes extremely low. A magnetic disk made of the polisheddisk is useful as a low-flying head type hard disk capable of achievinghigh-density storage.

A magnetic disk thus polished is very useful particularly inhigh-density storage media (having a storage density of 3 Gbits/inch² orgreater) represented by media that uses MR heads which utilize themagnetoresistance (MR) effect in magnetic disks, and is also useful fromthe standpoint of giving media with lower storage densities highreliability.

1. A polishing composition comprising abrasive grains, aphosphorus-containing inorganic acid or salt thereof and anotherinorganic acid or salt thereof contained in an aqueous medium.
 2. Thepolishing composition according to claim 1, wherein the abrasive grainsare at least one selected from the group consisting of alumina, titania,silica and zirconia.
 3. The polishing composition according to claim 1or 2, wherein the mean particle size of the abrasive grains is from0.001 to 0.5 μm.
 4. The polishing composition according to any of claims1 through 3, wherein the abrasive grains are colloidal particles.
 5. Thepolishing composition according to any of claims 1 through 4, whereinthe phosphorus-containing inorganic acid is phosphoric acid orphosphonic acid.
 6. The polishing composition according to any of claims1 through 5, wherein the other inorganic acid or salt thereof is atleast one acid selected from the group consisting of nitric acid,sulfuric acid, amidosulfuric acid and boric acid.
 7. The polishingcomposition according to any of claims 1 through 6, wherein theoxidizing agent is at least one compound selected from the groupconsisting of the peroxides, perborates, persulfates or nitrates.
 8. Thepolishing composition according to claim 7, wherein the peroxide ishydrogen peroxide.
 9. The polishing composition according to claim 7,wherein the perborate is sodium perborate.
 10. The polishing compositionaccording to any of claims 1 through 9, wherein the pH is 1 to
 5. 11.The polishing composition according to any of claims 1 through 10,wherein the abrasive grain content is in the range from 3 to 30% bymass, the content of the inorganic acid or salt thereof is in the rangefrom 0.1 to 8% by mass and the oxidizing agent content is in the rangefrom 0.2 to 5% by mass.
 12. The polishing composition according to anyof claims 1 through 11, wherein the polishing composition is acomposition for polishing magnetic disk substrates.
 13. A magnetic disksubstrate polished by the composition for polishing magnetic disksubstrates according to claim 12.